ncdxf     iaru


 

The Beacon Controller

Most of the hardware for each beacon is standard commercial equipment, but the beacon controller is specially designed. The beacon controller includes an Intel 8748 microprocessor, so to understand how the beacon controller works, you may need to look at both the hardware and the software for the microprocessor. Both were designed by Bob Fabry, N6EK.

It is improbable that anyone else would want to duplicate the beacon controller exactly. Substantial parts of the design deal with the precise timing of the transmissions needed for rapid frequency sharing and with the power stepping. If you don't want to do both, a simpler controller will suffice. In fact, most simple beacons use a programmable keyer as a controller.

Never-the-less, there has been a fair amount of interest in how the NCDXF beacon controller actually works, so we are including here the schematic and the source code. You have our permission to use whatever part of this would be useful to you. It would be fun for us if you let us know what you are doing.

The schematic is a PDF file, so you will need the Adobe Acrobat® Reader to view it.

If you want to calculate the exact timing of each beacon transmission, here is the information you need. Each code element or dot time is 54 milliseconds long. This corresponds to 22.22 words per minute using the normal computation. Dashes are three elements, spaces between dots and dashes are one element, letter spaces are three elements and word spaces are seven elements. The rig is nominally keyed at the start of the appropriate ten second period. The callsign is sent, followed by a word space, followed by four one-second dashes separated by letter spaces.

What are the uncertainties in this timing? The uncertainty in the start of the transmission is the sum of several delays: At the beginning of every second, the GPS receiver signals the controller using the interrupt line. The accuracy of this pulse is plus or minus one microsecond, according the specification of the GPS receiver. The pulse triggers an interrupt and the interrupt routine takes between thirteen and twenty-four 2.5 microsecond cycles to operate. This introduces a 42.5 to 60 microsecond delay. The main program is looping near the program label "xmi4" and will take twelve to sixteen cycles or 30 to 40 microseconds before keying the transmitter.  Thus the total delay before the TS-50 is keyed will be between 72.5 and 100 microseconds. Once the TS-50 is keyed, there is a delay of about 20 milliseconds before the RF actually appears at the feedline. This delay may vary a few milliseconds from one TS-50 to another, but should be repeatable for a single TS-50.

Once the transmission begins, the timing is controlled by the clock rate of the 8748 microprocessor. This clock rate is determined by a cheap crystal which should be within about one part in a thousand of the nominal frequency and should be stable to at least one part in one hundred thousand from day to day.